The present embodiment relates generally to optical sensors for measuring distances to objects (targets).
While it is known to transfer light through optical fibers, precision can be compromised due to the environmental effects on the fiber itself. These environmental effects can change the optical path length and the polarization of the light in the fiber, and can adversely affect measurement precision. The use of optical heterodyne detection can allow for optical radiation detection at the quantum noise level. As such, coherent optical systems can provide improvements in range, accuracy, reliability, scanning range, working depth of field, and operation in ambient light conditions. Furthermore, a coherent system can obtain sufficient information about the characteristics of a target location quickly.
Optical heterodyne detection includes a source light beam which is directed to a target and reflected therefrom. The return light beam is then mixed with a local oscillator light beam on a photo detector to provide optical interference patterns which may be processed to provide detailed information about the target. Optical heterodyne techniques can take advantage of the source and reflected light beam reciprocity. For example, these light beams can be substantially the same wavelength and are directed over the same optical axis. In this case, the signal-to-noise ratio (SNR) is sufficiently high so that a small receiving aperture may be used, for example, a very small lens capable of being inserted into limited access areas. Since a small receiver aperture can provide detailed information about the target, the optical components of a coherent system may be made very small.
Precision FM laser radars can incorporate a single chirp laser source and a polarization maintaining fiber optic geometry with separate local oscillator (LO) and signal paths. What is needed is a counter-chirp configuration that is made insensitive to vibration induced range errors by an accurate Doppler correction. What is further needed is combining the LO and signal paths for two lasers into a single fiber, so that the fiber optic circuit is less complicated, less expensive due to fewer components, and immune to error caused by changes in the LO and signal path lengths that result from environmental factors such as temperature changes. For example, the manufacturing industry, in which both background vibrations and changing environmental conditions exist, could be a candidate user for this laser configuration. The combination of LO and signal paths can provide the additional benefit that the sensor head portion of the unit can be placed in areas of restricted volume since it can be remoted arbitrarily far from the rest of the unit.
In summary, what is needed is a practical optical precision measurement system capable of great accuracy, rapid measurement time, access to tight spaces, flexibility, and reliability.